1. Field of the Invention
The present invention pertains to a method and apparatus for monitoring gas, and, in particular, to a sample cell for use in such a system and method.
2. Description of the Related Art
It is well-known to those skilled in the art that non-dispersive infrared (NDIR) type gas analyzers operate on the principle that the concentration of specific gases, such as carbon dioxide, nitrous oxide and anesthetic agents, can be determined by (a) directing infrared radiation from an infrared emitter through a sample of a gaseous mixture, (b) filtering this infrared radiation to minimize the energy outside the band absorbed by the specific gases, (c) measuring the radiation impinging upon a infrared radiation detector and which has passed through this sample, and (d) relating a measure of the infrared absorption of the gas to a gas concentration. Gases that may be measured exhibit increased absorption (and reduced transmittance) at specific wavelengths in the infrared spectrum. Moreover, the greater the gas concentration, the greater the absorption and the lower transmittance.
NDIR gas analyzers are widely used in medical applications and can be characterized as either in the main path of the patient's respiratory gases, known as mainstream or non-diverting gas analyzer, or located off of the main path, known as sidestream or diverting gas analyzer. Regardless of whether the analyzer diverts gas or not, the gas to be analyzed must transverse a flow passage in which infrared radiation passes through the gas sample. This portion of the passage, known as a sample cell, confines a sample composed of one or more gases to a particular flow path that is traversed by the optical path between the infrared radiation emitter and the infrared radiation detector. The infrared radiation emitter and an infrared radiation detector are both components of a transducer that may be detachably coupled to the sample cell. Other terminology, such as cuvette or airway adapter, is often used interchangeably with the term sample cell.
Mainstream designs require the optical and/or electronic components to be interfaced to the subject's airway or respiratory circuit. A mainstream analyzer is typically situated such that the patient's inspired and expired respiratory gases pass through the sample cell onto which a transducer, which includes elements necessary for monitoring respiratory gases, may be placed.
On the other hand, in sidestream analyzer designs, the optical and electronic component are typically positioned at a distance away from the subject's airway or respiratory circuit in communication therewith. U.S. Pat. No. 5,282,473, issued to Braig et al., discloses an exemplary sidestream infrared gas analyzer and sample cell. Sidestream gas analyzers typically communicate with a patient's airway by way of a long sampling plastic tube connected to an adapter, e.g., a T-piece at the endotracheal tube or mask connector, positioned along a breathing circuit or a nasal catheter that has been placed in communication with the patient's airway. When positioned along a breathing circuit, the sampling ports used by sidestream sampling systems are typically located in a wall of a component of the breathing circuit. The location of the sampling port along the breathing circuit may range anywhere from an elbow connected to an endotracheal tube to a wye connector at the opposite end of a breathing circuit. Gas passing through the sidestream analyzer is either exhausted to atmosphere or returned to the mainstream respiratory circuit.
As the patient breathes, gases are continuously drawn at sample flow rates ranging from 50 to 250 ml/min from the breathing circuit through the sampling tube and into the sample cell located within or near the monitor. To reduce the time delay associated with the transport of the gas sample through the sampling tube, conventional sidestream systems drawn gas at sample rates of 180 ml/min or higher. However, it is desirable to remove as little gas as possible from the breathing circuit and at the same time faithfully reproduce the gas waveform. Therefore, lower flow rates such as 50 ml/min have been used. In order not to degrade performance, as measured by accuracy of the end-tidal gas values, particularly at higher respiratory rates that may be seen in infants, careful attention must be paid to the total flow path from sampling site to the sample cell. While issues, such as mechanical mixing of the sample during transport in the sample line, and diffusion of the waveform within the sample line during transport, can be addressed by the selection of smaller bore tube, the proper design of the flow path through the sample cell is critical to achieve optimal performance. Present designs of sample cells have sample cell chamber volumes that are too large and/or inlet and outlets that tend to distort the profile of the waveform.
A typical sample cell is molded from an appropriate polymer, and has a passage defining the flow path for the gases being monitored. Typically, the optical path traverses the flow path with optical apertures in the wall of the sample cell and aligned along and on opposite sides of the flow passage. This configuration allows the beam of infrared radiation to enter the sample cell, traverse the gases in the flow passage, and, after being attenuated, exit from the sample cell to the filter and radiation detector. Transmissive windows in the optical apertures confine the gases to the sample cell flow passage and keep out foreign matter, while minimizing the loss of infrared energy as the beam enters and exits from the sample cell. The distance transversed by the infrared radiation in the flow passage of the sample cell is known as the measurement pathlength and is typically the distance between the optical apertures of the sample cell.
At a constant partial pressure or concentration of a gas, as the distance between the optical apertures increases, a greater quantity of the emitted infrared radiation is absorbed at the wavelength(s) specific to the gas of interest due to the presence of a greater number of molecules of the gas of interest. With a ratiometric measurement approach, emitted radiation at another wavelength, where little or no absorption occurs, is concurrently measured, and the ratio of the measurements at the two wavelengths allows the partial pressure of the gas to be determined. Thus, to achieve an acceptably quiet (noise free) and a fast response time for the gas analyzer, both the measurement pathlength and volume of the sample cell must be carefully considered.
Therefore, a need exists for a sample cell that has a low internal volume, minimizes unswept volume, and allows for sufficient pathlength so that an acceptable signal can be measured. Such a design must provide efficient “flushing” of the volume so that the response time is not compromised. Additionally, this needs to be achieved in a manufacturable and commercially viable manner.